Is MR (Magnetic Resonance) venography effective in diagnosing herpes encephalitis?

MR Venography Is Not Recommended for Diagnosing Herpes Simplex Encephalitis

MR venography is not an effective diagnostic tool for herpes simplex encephalitis (HSE); instead, standard MRI with diffusion-weighted imaging is the preferred imaging modality for suspected HSE cases. 1

Recommended Diagnostic Approach for HSE

Initial Imaging

• MRI (including diffusion-weighted imaging) is the preferred imaging modality and should be performed within 24-48 hours of hospital admission for all patients with suspected encephalitis 1
• Early MRI changes in HSE occur in the cingulate gyrus and medial temporal lobe, showing gyral edema on T1-weighted images and high signal intensity on T2-weighted and FLAIR images 1
• MRI is abnormal in approximately 90% of HSE patients within 48 hours of hospital admission, making it significantly more sensitive than CT scanning 1
• Diffusion-weighted MRI may be especially sensitive for detecting early changes in HSE 1

When MRI Cannot Be Performed

• If the patient's condition precludes an MRI (e.g., severe agitation, confusion), urgent CT scanning may be used to exclude structural causes of raised intracranial pressure or reveal alternative diagnoses 1
• However, CT has limited sensitivity - only about 25% of HSE patients show abnormalities on initial CT scan 1

Advanced Imaging Considerations

• MR spectroscopy, SPECT, and PET are not indicated in the assessment of suspected acute viral encephalitis 1
• MR venography is not mentioned in any of the guidelines as a recommended diagnostic tool for HSE 1

Diagnostic Gold Standard

• CSF PCR for HSV DNA is the diagnostic gold standard for HSE 1, 2
• However, CSF PCR can occasionally be negative in HSE, especially if performed early (<72 hours after symptom onset) or late in the illness 1, 2
• In cases with high clinical suspicion but negative initial PCR, treatment should continue and a repeat lumbar puncture with PCR testing should be performed 24-48 hours later 1

Treatment Considerations

• Intravenous aciclovir (10 mg/kg three times daily) should be started if initial CSF and/or imaging findings suggest viral encephalitis, or within 6 hours of admission if these results will not be available 1
• Treatment should not be delayed waiting for confirmatory tests, as delays in initiating aciclovir therapy are associated with poorer clinical outcomes 3, 4
• Treatment should continue for 14-21 days in confirmed HSE cases 1

Common Pitfalls to Avoid

• Relying on a single negative CSF PCR to rule out HSE - false negatives can occur, as demonstrated by a case with two negative PCRs that was later confirmed as HSE on autopsy 2
• Delaying treatment while waiting for imaging or laboratory confirmation - this is associated with increased mortality and morbidity 3
• Using CT as the only imaging modality - CT has poor sensitivity for early HSE changes 1
• Stopping aciclovir prematurely - treatment should only be discontinued if an alternative diagnosis is made or specific criteria are met 1

Historical Context

• Before the development of CSF PCR for HSV DNA, brain biopsy was the preferred method for diagnosing HSE 1
• With modern diagnostic techniques, brain biopsy now has no place in the initial assessment of suspected HSE 1, 5
• Brain biopsy should only be considered in patients with suspected encephalitis in whom no diagnosis has been made after the first week, especially if there are focal abnormalities on imaging 1

In summary, MR venography plays no role in the diagnosis of herpes simplex encephalitis. Standard MRI with diffusion-weighted imaging is the recommended imaging modality, complemented by CSF PCR testing for HSV DNA.